The present invention is directed to a shock-stabilized duct-mode device for creating a high temperature and high velocity flame jet suitable for spraying high melting point materials.
Flame jets are utilized for general heating purposes as well as specific uses including cutting and drilling of granite and the thermal spraying of metallic or other materials to form coatings on a base material. Where high heat transfer rates and/or supersonic velocity flame jets are required, certain types of flame-producing device have been available. These devices reduce to two basic modes of operation--the chamber-stabilized mode and the duct-stabilized mode.
The earliest description of both the duct and chamber modes is given in the G.H. Smith et al. patent (U.S. Pat. No. 2,861,900). FIG. 1a of the present application is a simplified sketch of a "duct stabilized" device of the type described by Smith et al. The burner 10 consists of two bores of different diameter. Oxygen enters the burner 10 through a relatively small diameter bore 12. Fuel, entering bore 12 through passage 13, mixes with the oxygen flow and the combined flow is discharged from bore 12 into the larger duct 11. The oxy-fuel mixture is ignited upon its entry to duct 11 with nearly complete combustion occurring prior to exit of the flame products from duct 11. Supersonic flame 14 extends as a flame-jet beyond duct 11 and is characterized by shock diamonds 16. Metallic powder is injected through duct 16.
In this conventional "duct mode" geometry (FIG. 1a) the gas flow is "choked". That is, the walls of duct 11 prevent the rapid expansion of the gas required to reach supersonic velocity. Supersonic velocity only occurs beyond the exit of duct 11 in the open atmosphere. In "choked flow" the gas pressure over the entire duct length remains above atmosphere (see FIG. 1b). In "choked flow" the exit gas velocity has reached sonic velocity (see Fe. 1c) which for the hot products of combustion is about 3,000 feet per second.
FIG. 3a of the present application is a simplified sketch of a "chamber-stabilized mode" of the type described by Smith et al. The "chamber stebilized mode" of FIG. 3a utilizes a relatively large volume chamber 31 to stabilize and contain the combustion reactions. Oxygen and fuel are fed under pressure into chamber 31 in burner 30 through ports 32 and 33. A very small nozzle throat 34 with an expanding conical bore 35 expands the hot gas exiting from chamber 31 to extremely high velocity. For an inlet oxygen pressure of 500 psig (FIGS. 1b and 1c) the exit gas velocity is over 8,000 ft/sec. Where high particle impact velocities are required for thermal spray process optimization, the "chamber mode" is superior to the "duct mode". However, as the oxygen pressure is raised to produce favorable particle velocities, unacceptable heat losses to the cooling water (not shown) occur. Higher melting point materials such as aluminum oxide remain solid and will not form a coating.
The "duct mode", with a much smaller "wetted surface" available for heat transfer from the flame to the cooling water (not shown) has much higher flame-jet temperatures than for the "chamber mode". Thus, even though particle velocities are much lower, it may have to be selected for certain types of thermal spraying.
Another form of duct-stabilized device for limiting particle build-up on the inner duct walls is disclosed in the Browning patent (U.S. Pat. No. 4,836,447). In this patent, the expanding section 12 acts as a diffuser and at no point along the path of the gas stream is the flow supersonic.